Walnut Tree

Walnut trees are large trees in the genus Juglans that give us tasty walnuts and some of the most beautiful wood in the world. The best known are the black walnut of eastern North America and the English (or Persian) walnut, which produces most of the walnuts we buy in shops.

Scientific Classification & Quick Facts

Common name	Walnut
Scientific name	Juglans
Kingdom	Plantae
Phylum	Tracheophyta
Class	Magnoliopsida
Order	Fagales
Family	Juglandaceae
Type	Large deciduous hardwood nut tree
Habitat	Deep, moist, well-drained loamy soil in full sun
Native range	Mostly the United States and the Americas
Lifespan	Up to 200 years
Height	Up to 100 feet
Edible parts	Nuts, dried before storing
Special feature	Roots and leaves release juglone, which harms nearby plants

Why Walnut Trees Are Special

Walnut is one of the most valuable hardwoods. Its rich, dark brown wood is prized for furniture, flooring, cabinets, carvings and gunstocks. The nuts are full of healthy fats and protein, and people have eaten them for thousands of years.

Walnut trees also make great shade trees, and wildlife loves them. Squirrels, birds and deer all visit walnut trees for food and shelter, so some landowners plant them just to attract animals.

The genus name Juglans comes from Latin words meaning “Jupiter’s nut,” because the ancient Romans thought walnuts were fit for the king of the gods.

The Walnut’s Secret Weapon: Juglone

Have you ever noticed that some plants turn yellow and wilt when they grow under a black walnut? That’s because the tree makes a chemical that harms many neighboring plants.

Black walnut roots, leaves, bark, stems and nut husks contain a colorless, harmless substance called hydrojuglone. When it is exposed to air or soil, it changes into juglone, a chemical that is toxic to many plants, such as tomatoes, potatoes, blueberries and azaleas. Plants that harm their neighbors with chemicals are called allelopathic.

Black walnut makes the most juglone. Its relatives, including butternut, English walnut, pecan and hickories, make smaller amounts. A few other trees, such as tree-of-heaven and black locust, also release chemicals that can affect nearby plants.

What Walnut Trees Look Like

Walnut trees are big. A mature black walnut often grows 50 to 100 feet (15 to 30 meters) tall with a wide, rounded crown, while English walnuts are usually a bit smaller. Walnut trees can live for 150 years or more.

• Leaves: Walnut leaves are compound, which means each leaf is made of many small leaflets along one central stalk. A black walnut leaf can have 15 to 23 leaflets. In autumn the leaves turn yellow and drop early.
• Flowers: In spring, walnut trees have long, drooping, greenish male flowers called catkins and small, separate female flowers that become nuts. The wind carries the pollen.
• Fruits: Each nut grows inside a thick green husk. Inside the husk is the hard, wrinkly, brown shell, and inside that is the kernel we eat. Black walnut husks turn dark brown to black as they ripen and stain everything they touch.
• Bark: Mature black walnut bark is dark brown to gray-black, with deep ridges that form a diamond-shaped pattern.

Young walnut trees grow fairly fast and can reach around 25 feet (7.5 meters) in about 10 years in good conditions.

Where Walnuts Grow

Wild black walnuts grow in forests across the eastern and central United States. The English walnut comes originally from Central Asia and the region around Iran, and it has been grown across Europe and Asia for thousands of years.

Today, China and the United States (mostly California) grow the most walnuts. Other important walnut-growing countries include:

• Turkey
• Iran
• Chile
• Ukraine
• France
• Italy
• Romania
• India

Growing a Walnut Tree at Home

Soil and Site

Walnut trees grow best in deep, rich, well-drained loamy soil that holds moisture. They like a pH around 6.0 to 7.0 but can handle anything from about 5 to 8.

Choose a spot with at least 3 feet (about 1 meter) of good soil and full sun. Avoid shallow, rocky or heavy clay soils, steep dry slopes, and low spots where frost or water collects. In hilly areas, the lower parts of north- or east-facing slopes, where the soil stays moist, are often good. Remember that walnuts get huge and drop messy nuts, so plant them well away from houses, driveways and vegetable gardens.

Planting

• From nuts: In fall, plant fresh nuts 2 to 3 inches (5 to 7.5 cm) deep. Plant several, because squirrels may dig some up. Seedlings that come up can be thinned later.
• Young trees: Walnuts grow a long taproot, so small young trees (under about 10 feet or 3 meters) transplant best. Avoid trees that have been in a pot so long that their roots are circling.
• Spacing: Full-grown trees need lots of room. Space them about 40 to 60 feet (12 to 18 meters) apart.
• Pollination: Walnuts are partly self-fertile, but planting two different varieties usually means a bigger crop.

Watering

Young walnut trees need regular water. Give them a deep soak about once a week, and more often in hot, dry summer weather. Water deeply rather than lightly so the roots grow down.

Older trees have deep roots and need less help, but they still produce better nuts if they get water during long dry spells. Keep grass and weeds away from the base of young trees and add a layer of mulch to hold in moisture.

Care

• Feed young trees in early spring with a balanced fertilizer or well-rotted compost, following the package directions. Don’t overdo it.
• Newly planted trees may need a stake for support and extra water in dry spells during their first couple of years.
• Spread compost or wood-chip mulch around the base, keeping it a few inches away from the trunk.
• Prune in winter when the tree is dormant to remove dead, damaged or crossing branches.

Pests and Diseases

Walnut trees are fairly tough, but a few problems can affect them:

• Squirrels: They can eat a whole crop. A smooth metal or plastic collar around the trunk and trimming low branches can make climbing harder.
• Codling moths and walnut husk flies: Their larvae damage the nuts and husks.
• Walnut leaf gall mites: These cause small blisters or pouches on the leaves but rarely harm the tree.
• Walnut blight, anthracnose and leaf blotch: These diseases cause dark spots on leaves and nuts. Raking up and removing fallen leaves in autumn helps stop the disease from coming back.

Harvesting and Storing Walnuts

Walnuts are ready in autumn when the husks start to split or the nuts fall from the tree. Collect them every day if you can, before squirrels and birds get them.

• First, crack open a few nuts to make sure the kernels are fully developed.
• Wear gloves when you remove the husks, because walnut juice stains skin dark brown for days.
• Remove the husks soon after harvest. Leaving them on can make the nuts moldy and spoil the flavor, especially once the husks turn black and mushy.
• Scrub the shells and spread the nuts out to dry in a warm, airy spot out of direct sun. If using an oven or dehydrator, keep the temperature below about 104°F (40°C).
• When the kernels snap cleanly, the nuts are dry. Store them in a cool, dry place, or shell them and keep the kernels in the fridge or freezer.

In places where summers are too cool for walnuts to ripen fully, people sometimes pick them green in early summer, before the shell hardens, and make pickled walnuts, a traditional treat in Britain.

Types of Walnut Trees

There are around 20 species of walnut trees around the world. Six grow wild in the United States. Some of the best known are:

• Black walnut (Juglans nigra)
• English or Persian walnut (Juglans regia)
• Butternut (Juglans cinerea)
• Arizona walnut (Juglans major)

Other walnut species include:

• Northern California black walnut
• Southern California black walnut
• Little walnut
• Japanese walnut
• Manchurian walnut
• Andean walnut
• Bolivian walnut
• West Indian walnut

Walnut FAQs